The present invention relates to a process for producing rare earth iron-based sintered permanent magnets of high performance, which predominantly comprise one or more rare earth metals, boron, and iron (or iron and cobalt), and to a powder mixture for use in compaction to produce rare earth iron sintered permanent magnets by such a process.
Permanent magnets are one class of important materials commonly incorporated in electrical or electronic equipment and are widely used in various apparatuses ranging from household appliances to peripheral equipment for supercomputers. Due to a continuing demand for electrical and electronic equipment having a reduced size and improved performance, permanent magnets are also required to have improved performance.
The magnetic performance of a permanent magnet is normally evaluated by intrinsic coercive force (iHc), residual flux density (Br), and maximum magnetic energy product [(BH)max], all of which should be as high as possible. These magnetic properties are hereinafter referred to as "magnet properties".
Typical conventional permanent magnets are Alnico, hard ferrite, and rare earth cobalt magnets. Due to recent instability of the cobalt supply, the demand for Alnico magnets has been declining since they contain on the order of 20%-30% by weight of cobalt. Instead, inexpensive hard ferrite, which predominantly comprises iron oxide, has tended to be predominantly used as a material for permanent magnets.
Rare earth cobalt magnets are very expensive since they comprise about 50%-60% by weight of cobalt and contain samarium (Sm) which is present in a rare earth ore in a minor proportion. Nevertheless, such magnets have increasingly been used, mainly in compact magnetic circuits of high added value, in view of their magnet properties, which are significantly superior to those of other magnets.
Recently developed permanent magnets are rare earth iron magnets, which are less expensive than rare earth cobalt magnets since they need not contain expensive samarium or cobalt and yet exhibit good magnet properties. For example, a permanent magnet made of a magnetically anisotropic sintered body comprising a rare earth metal (REM), iron, and boron is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 59-46008(1984). A similar magnetically anisotropic sintered permanent magnet in which iron is partially replaced by cobalt such that the resulting alloy has an increased Curie point so as to minimize the temperature dependence of magnet properties is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 59-64733(1984).
These magnets, which comprise REM, Fe, and B, or REM, Fe, Co, and B, are hereinafter referred to as R--Fe--B magnets, in which R stands for at least one element selected from rare earth elements including yttrium (Y), and part of Fe may be replaced by Co. Magnetically anisotropic R--Fe--B permanent magnets exhibit, in a particular direction, excellent magnet properties which are superior even to those of the above-mentioned rare earth cobalt magnets.
R--Fe--B sintered permanent magnets are usually produced by melting constituent metals or alloys (e.g., ferroboron) together to form a molten alloy having a predetermined composition, which is then cast to form an ingot. The ingot is crushed to an average particle diameter of 20-500 .mu.m and then finely ground to an average particle diameter of 1-20 .mu.m to prepare an R--Fe--B alloy powder to be used in compaction.
Alternatively, an R--Fe--B alloy powder may be directly prepared by the reduction diffusion method in which a mixture of a rare earth metal oxide powder, iron powder, and ferroboron powder is reduced with granular calcium metal and the reaction mixture is treated with water to remove calcium oxide formed as a by-product. In this case, the resulting alloy powder may be finely ground to an average particle diameter of 1-20 .mu.m, if necessary.
Since the R--Fe--B alloy has a main crystal structure of the tetragonal system, it can readily be finely divided to form a fine alloy powder having a relatively uniform size. The finely ground alloy powder is compacted by pressing (compression molding) while a magnetic field is applied in order to develop magnetic anisotropy, and the green powder compacts formed are sintered to give sintered permanent magnets, which may be subjected to aging after sintering. If desired, the sintered magnets may be plated with an anticorrosive film of Ni or the like in order to provide the magnets with improved corrosion resistance.
It is described in Japanese Patent Applications Laid-Open Nos. 63-317643(1988) and 5-295490(1993) that a molten R--Fe--B alloy is rapidly solidified by the twin or single roll method to form a thin sheet or thin flakes having a thickness of 0.05-3 mm and consisting of fine grains in the range of 3-30 .mu.m. The thin sheet or flakes are crushed and finely ground to be used in the production of sintered magnets. The resulting sintered magnet has further improved magnet properties, particularly in maximum energy product [(BH)max].
In compression molding of an alloy powder to produce a magnetically anisotropic sintered magnet, a small proportion of a lubricant is normally added to the powder in order to ensure mobility of the alloy powder during compaction and facilitate mold release. If the mobility is not sufficient, friction between the powder and the mold such as the die wall exerted during compression may cause flaws, delaminations, or cracks to occur on the surface of the die or green compact, and rotation of the powder is inhibited. Such rotation is required to align the readily magnetizable axes of individual particles of the alloy powder along the direction of the applied magnetic field so as to develop magnetic anisotropy.
Various substances have been proposed as lubricants for use in compaction of an R--Fe--B alloy powder for use in the production of sintered magnets. Examples of such substances include higher fatty acids such as oleic acid and stearic acid and their salts and bisamides as described in Japanese Patent Applications Laid-Open Nos. 63-138706(1988) an 4-214803(1992), higher alcohols and polyethylene glycols as described in Japanese Patent Application Laid-Open No. 4-191302(1992), polyoxyethylene derivatives such as fatty acid esters of a polyoxyethylene sorbitan or sorbitol as described in Japanese Patent Application Laid-Open No. 4-124202(1992), a mixture of a paraffin and a sorbitan or glycerol fatty acid ester as described in Japanese Patent Application Laid-Open No. 4-52203(1992), and solid paraffin and camphor as described in Japanese Patent Application Laid-Open No. 4-214804(1992).
It is described in Japanese Patent Application Laid-Open No. 4-191392(1992) that a lubricant such as a higher fatty acid or polyethylene glycol is added to an R--Fe--B alloy powder during fine grinding so as to coat the alloy powder with the lubricant in a dry process.
However, the lubricating effects of conventional lubricants are not very high, so it is necessary to apply a mold release agent such as a fatty acid ester to the mold or add a lubricant to the alloy powder in a large proportion in order to prevent the occurrence of flaws or the like on the surface of the die or the green compacts. Application of a mold release agent makes the compacting procedure complicated, thereby significantly interfering with the production efficiency of continuous mass production of sintered magnets. Addition of a lubricant in a large proportion results in an increased residual carbon content of the magnets formed after sintering, thereby adversely affecting the magnet properties, particularly intrinsic coercive force (iHc) and maximum energy product [(BH)max]. In addition, due to the extremely high tendency for agglomeration, the lubricant is present as agglomerated masses even after being mixed with the alloy powder, and this leaves large voids which cause pinholes to form when the sintered magnets are finally coated with an anticorrosive film. If the lubricating effect is insufficient, the alloy powder is prevented from rotating during compaction in a magnetic field, thereby adversely affecting the alignment of the powder and hence the residual flux density (Br) of the resulting magnet.